Earthing

[ThrownRazor25]

Went to a job today regarding high ze.there is no earth at the house itself it is relying on water pipe for an earth.i put on earth rod in and got my ze down to 51 ohms.the main board tho is reviewable fuses am I right that if I changed the board to newer one with rcd protection I am allowed 1667 ohms as a maximum zs on my circuits?

 

[telectrix]

if all circuits are protected by a 30mA RCD, then 1667 is your max. figure. however, with a bit more effort and a couple or so decent rods, you should be able to get a more acceptable Ze ( Ra).

 

[Leesparkykent]

Heres a good read to help understand why

http://www.napitonline.com/downloads/CP 7 07 P 8-10 16th TT Design and Test.pdf

 

[Octopus]

Went to a job today regarding high ze.there is no earth at the house itself it is relying on water pipe for an earth.i put on earth rod in and got my ze down to 51 ohms.the main board tho is reviewable fuses am I right that if I changed the board to newer one with rcd protection I am allowed 1667 ohms as a maximum zs on my circuits?

In a word YES, BUT..... if you are 100% sure that the RCD won't fail, so always endeavour to get the Zs for the final circuits within the maximum allowed for the value of the MCB.

Going around using 1667 is simply a cop out.

Obviously a TT system is different, and you should still do the job properly and determine the Ze, do a R1 + R2, and then do the Zs to check.

JMO

 

[davesparks]

Went to a job today regarding high ze.there is no earth at the house itself it is relying on water pipe for an earth.i put on earth rod in and got my ze down to 51 ohms.the main board tho is reviewable fuses am I right that if I changed the board to newer one with rcd protection I am allowed 1667 ohms as a maximum zs on my circuits?

If one rod achieves 51 ohms then a proper system of electrodes should get a pretty good result. I'm assuming that by one rod you mean a ridiculous 4'x3/8" affair?

 

[valleybilly]

Heres a good read to help understand why

http://www.napitonline.com/downloads/CP 7 07 P 8-10 16th TT Design and Test.pdf

Fig 2 shows Ze test on N PMSL weldone Napit !

 

[telectrix]

Fig 2 shows Ze test on N PMSL weldone Napit !

well spotted that man. coconut but no cigar. smoking in workplace ist verboten.

 

[rapparee]

In a word YES, BUT..... if you are 100% sure that the RCD won't fail, so always endeavour to get the Zs for the final circuits within the maximum allowed for the value of the MCB.

Going around using 1667 is simply a cop out.

Obviously a TT system is different, and you should still do the job properly and determine the Ze, do a R1 + R2, and then do the Zs to check.

JMO

I thought <200 was required for a tt system.

 

[telectrix]

I thought <200 was required for a tt system.

no. according to BS7671,a value above 200 is liable to be unstable. there's no max. value stipulated.

 

[shanky887614]

no. according to BS7671,a value above 200 is liable to be unstable. there's no max. value stipulated.

technically it could be 700 ohms and show infinity on some testers and show a reading on others

some old testers only go to say 299 for example.

technically 1779 is the limit for a 30ma rcd and 100 if memory is correct for a 500ma but i would have to check

 

[malcolmsanford]

To be honest guys I'm fully appreciative that getting a rod down to it's lowest value is preferred but with all TT systems there is only so much you can do, and you really are relying on the RCD to provide the protection you want.

Even if you get the rod(s) down to 5 ohms say any 'B' breaker over 6 amps and it is virtually impossible to achieve the Zs value for that breaker, and there isn't a cats hell in chance with 'C' or 'D' breakers.

By all means try and achieve a stable reading for the rod(s), but that is all you really trying to achieve, stability. Anything 50 and below is really quite acceptable. The BS 7430 gives us a guide of anything under 100 ohms which is the NICEC take on it as well.

 

[HappyHippyDad]

no. according to BS7671,a value above 200 is liable to be unstable. there's no max. value stipulated.

Now that is interesting..and a little scary! I'm afraid I dont feel quite as strongly (either rightly or wrongly)as some of the time served about getting the Ze down to below 10 on a TT when fault protection is from the RCD. I just cant see what that is actually achieving and I've never been given a solid response even from Davesparks and E54 who seem to have the most knowledege regarding Earth rods.

A value of 200Ω or below is chosen to achieve a degree of stability (i.e so that it will not rise above 1667 even in dry conditions) so that a 30mA RCD will be effective for fault protection.

I can see how getting the Ze < 1Ω would be effective but not anything greater as the cut out fuse would not disconnect in the required times (if at all) so whatever the figure you are relying solely on the RCD. However, relying on a figure of 1667Ω doesn't make much sense to me as that figure will not be stable and will rise above that value... so I guess I'll go with a maximum Ze of 200Ω as suggested by GN3.

 

[Leesparkykent]

Now that is interesting..and a little scary! I'm afraid I dont feel quite as strongly (either rightly or wrongly)as some of the time served about getting the Ze down to below 10 on a TT when fault protection is from the RCD. I just cant see what that is actually achieving and I've never been given a solid response even from Davesparks and E54 who seem to have the most knowledege regarding Earth rods.

A value of 200Ω or below is chosen to achieve a degree of stability (i.e so that it will not rise above 1667 even in dry conditions) so that a 30mA RCD will be effective for fault protection.

I can see how getting the Ze < 1Ω would be effective but not anything greater as the cut out fuse would not disconnect in the required times (if at all) so whatever the figure you are relying solely on the RCD. However, relying on a figure of 1667Ω doesn't make much sense to me as that figure will not be stable and will rise above that value... so I guess I'll go with a maximum Ze of 200Ω as suggested by GN3.

<1Ω is quoted for the Maximum Ra of the earth electrode/s by some of the members as they don't like relying on an RCD for earth fault protection and also see that using RCDs as a substitute doesn't make good practice due to failures. I've read a report that stated out of 607 RCDs tested it was recorded that the failure rate was 3.8%.


Obviously a stable electrode <1Ω would let you achieve a low enough impedance to enable a large enough fault current to flow to operate the OCPD within the specified time.Where these values cannot be achieved or where there is some doubt about their stability, then an alternative method is required. It is in this situation that the residual current device offers the most practical solution because it has the ability to operate on circuits having much higher values of earth fault loop impedance. The basis of RCD protection in this situation is to ensure that any voltage, exceeding 50V that arises due to earth fault currents, is immediately disconnected. This is achieved by choosing an appropriate residual current rating and calculating the maximum earth loop impedance that would allow a fault voltage of 50V (RA x I∆n d 50V).


Some times (especially in a domestic setting) getting a <1Ω Ra value is just unachievable (Tin hat on ready)! IMO The resistance of the installation earth electrode/s should be as low as practicable and If TN values are unachievable and you're solely relying on an RCD as earth fault protection then it always prudent to install a 100ma type S RCD upfront to provide addition protection incase of a faulty/failed 30ma RCD.

 

[davesparks]

Now that is interesting..and a little scary! I'm afraid I dont feel quite as strongly (either rightly or wrongly)as some of the time served about getting the Ze down to below 10 on a TT when fault protection is from the RCD. I just cant see what that is actually achieving and I've never been given a solid response even from Davesparks and E54 who seem to have the most knowledege regarding Earth rods.

A value of 200Ω or below is chosen to achieve a degree of stability (i.e so that it will not rise above 1667 even in dry conditions) so that a 30mA RCD will be effective for fault protection.

I can see how getting the Ze < 1Ω would be effective but not anything greater as the cut out fuse would not disconnect in the required times (if at all) so whatever the figure you are relying solely on the RCD. However, relying on a figure of 1667Ω doesn't make much sense to me as that figure will not be stable and will rise above that value... so I guess I'll go with a maximum Ze of 200Ω as suggested by GN3.

Stability is achieved through depth and quantity of electrodes.

If a single 4' rod is used and this gets say 190ohms Ra then anyone following the 200ohms theory will accept it. However the first 2/3' of the ground will suffer the most seasonal variations due to moisture levels and changes in the water table etc, so that the value will not be stable and you can expect wild variations.
If a 12' rod is used and this also gives a 190ohm Ra then this will be far more stable as it has much greater depth.

Unfortunately the vast majority of people in the job these days appear to be far to stupid to understand that soil conditions vary from place to place and so the stability of an Ra value varies from place to place. It is entirely possible that a value of 500ohms in one place is far more stable than a value of 200ohms in another place.

Hence the best advice that can be given is to get the Ra as low as possible using multiple 5/8" rods and not just use a single rod and hope it works.

 

[HappyHippyDad]

Stability is achieved through depth and quantity of electrodes.

If a single 4' rod is used and this gets say 190ohms Ra then anyone following the 200ohms theory will accept it. However the first 2/3' of the ground will suffer the most seasonal variations due to moisture levels and changes in the water table etc, so that the value will not be stable and you can expect wild variations.
If a 12' rod is used and this also gives a 190ohm Ra then this will be far more stable as it has much greater depth.

Unfortunately the vast majority of people in the job these days appear to be far to stupid to understand that soil conditions vary from place to place and so the stability of an Ra value varies from place to place. It is entirely possible that a value of 500ohms in one place is far more stable than a value of 200ohms in another place.

Hence the best advice that can be given is to get the Ra as low as possible using multiple 5/8" rods and not just use a single rod and hope it works.

Thanks for the explanation Dave.

One last question on the matter.

I know you would not be happy with an Ra of 200Ω, but could you answer if you believe that a value of 200Ω (even if achieved through one 3/8" rod) could ever increase above 1667Ω? Have you ever known such a large change to occur? if you have would you say this is extremely unlikley or quite possible?


If such a large variance is pretty much impossible then would you say 200Ω will offer the same degree of security as >1Ω on a TT installation which is protected by an RCD, because both figures will not allow a large enough fault current to flow to operate the Supply fuse, and both figures will be sufficient to operate a 30mA RCD (limiting fault voltage to below 50V)?